The present invention relates generally to compositions of ethylene-propylene rubber adapted for use as insulation compositions and to conductors insulated with such compositions. More specifically the invention relates to an ethylene-propylene composition having a desired and needed combination of properties as an insulator including improved tensile properties in combination with heat resistant properties and to insulated conductors having such composition forming the insulation thereof.
It is well-known that polymeric compositions, when used for insulating purposes, are used in many different environments including temperature environments, and that no one composition is suitable for all uses. To a large degree the use, which is made of an insulating composition, places important requirements on the composition and there are many standards which have been established in the industry, and by standards organizations such as the Underwriters Laboratories, ASTM, IPCEA, for wire insulation compositions. In all of the standards which are established there is a close correlation between the requirements of the insulated composition and of the insulated wire or cable and the use to which the composition or cable is to be put. Accordingly, it is well-established in the wire and cable technology and industry that different combinations of insulating properties and physical properties are required in different cables where such cables are to be used in applications having particular criteria such as temperature criteria, atmospheric criteria, voltage criteria and other measurable criteria of compositions and of cables.
Another important criteria of insulating compositions, and of cables formed with such compositions, is the economic criteria or more specifically the ability to prepare such compositions and cables at reasonable costs.
In some other cases a composition can be prepared in a bulk form, or an extended form, or a sheet form and have good properties but the same properties are not retained when a product is put on a conductor to form a wire or cable. For still other compositions the process by which the material is applied to a conductor may give deleterious results in the finished product or may make the application process uneconomical.
Compositions which have been prepared in the past of ethylene-propylene rubber and which have very desirable heat resistance and other properties for use as wire and cable insulation are disclosed in U.S. Pat. Nos. 4,069,190 and 4,133,936, both patents being assigned to the same assignee as the subject application. As is brought out in these patents the insulating composition and the cable made with the insulating composition has a combination of properties, values of which are set forth in the data included in the patents. Further, the composition is made up of a number of ingredients which are set forth in range values also explicitly included in the data of the preferred embodiments.
One of the desirable properties which is achieved in the compositions and product of the patents referenced above is heat resistance.
In developing the composition, the heat resistance needed was not as great as that of silicone rubbers which had been used in prior compositions but the overall combination of properties including heat resistance was achieved at a cost substantially below the silicone rubbers. In other words the newer compositions of the reference patents were substituted for silicone rubber compositions previously employed as heat resistance wire insulation compositions.
One of the criteria in achieving heat resistance as is brought out in the patents is the avoidance of cracking and deterioration of the composition when employed at the elevated temperatures. The heat resistant properties of the composition of the reference patents are attributed to a number of factors. In this regard it is important to appreciate that a composition made up of such a combination of materials achieves a combination of properties based on the proper blending and curing of the components as prescribed in the patent. With regard to the heat resistant properties one of the elements or components which contributes to the development of the heat resistant property, but not the only component, is the inclusion of the antioxidants in the overall combination in the ranges set forth in the reference patents. Other components are deemed significant in achieving the overall beneficial combination of properties of the composition both by itself and also as a wire insulation. However, with reference to the antioxidant the combination of ingredients includes the antioxidant which is present in order to inhibit such reactions at elevated temperatures as cause deterioration of the composition and product. As is evident from the text of the prior art patents, there is also present in the overall composition a peroxide material which is added according to the prescribed method to provide the cross-linking as set out in the patent. Such cross-linking gives the compositions of the reference patents some of their higher temperature properties. It has generally been understood with respect to higher temperature compositions formed by peroxide curing that the presence of the peroxide and the presence of the antioxidant material can be antagonistic to each other in the sense that their functions in the overall composition are for inconsistent purposes. More specifically the peroxide material is present to induce cross-linking. By contrast the antioxidant material is present as generally contemplated to limit or inhibit the post-cured oxidation of the composition at the elevated temperatures at which it is used.
With reference to U.S. Pat. Nos. 4,069,190 and 4,133,936 the compositions and products are referred to as heat resistant and this heat resistance is indicated in the statement of the background of the invention to pertain to the loss of elasticity or to increase embrittlement upon exposure to temperatures above ambient temperatures. As stated in the patent, "The deteriorating effect of heat on elastomers has prompted continuing efforts and the use of a variety of remiedal measures to improve their resistance to heat, such as the development and use of antioxidant or agents which block the action of oxygen or free radical forming ingredients, and new compound formulations."
It is well known that different applications for wires require that the wires have different sets of properties depending on the application to be made of the wire.
For motor lead and apparatus lead applications the particular combination of properties which is required include a basic heat resistance of the insulation compound. For such applications a preferred combination includes an economical heat resistant, tough, flexible insulation with a moderate tensile strength and also with good tear resistance and good abrasion resistance. For such motor lead and apparatus lead applications the insulation jacket is integral in the sense that there is no outer jacket applied over the insulation jacket having these properties. In other words there is only a single insulating jacket applied and that jacket is the one which must have the desirable combination of properties recited above. Of course, the composition must also have a needed or standard set of electrical properties.
In general heat resistant properties of polymers may be increased by cross-linking of the polymer molecules. Such cross-linking can be accomplished for some polymer systems by use of thermally unstable peroxide compositions. However the aging properties of polymer systems at high temperature may be adversely affected by presence of peroxide curing agents and antioxidant compositions have been added to polymer systems to inhibit or overcome such deleterious effects.
Mechanisms for antioxidant use are discussed in the following exerpt from a booklet entitled "Handbook On Antioxidant And Antiozonants" for rubber and rubber like products published by Goodyear Chemicals. Two excerpts from page 11 of the booklet are as follows:
"By adding an antioxidant of one or more types, the oxidation of polymers can be interrupted and the degradation slowed down considerably. There are two ways this can usually be accomplished. The first way is to introduce an antioxidant that will eliminate peroxides before they can do damage. EQU ROO.degree.+AH.fwdarw.ROOH+A.degree. EQU ROO.degree.+A.degree..fwdarw.ROOA PA1 The antioxidants which will do this are the phenolics and the aromatic amines. The amines include most of the staining antioxidants and the antiozonants." PA1 "Another way of disrupting the oxidative action is to destroy the hydroperoxides before then can cause problems. EQU ROOH+AH.fwdarw.Stable Products PA1 Two types of antioxidants do this. They are the phosphites and the thioesters. The most common phosphite is trisnonylphenylphosphite and a well known thioester is dilaurylthiodipropionate, frequently called simply . . . DLTDP. PA1 The thioesters are most widely used in plastics, principally polyolefins, while the phosphites are used in the rubber industry almost exclusively as emulsion polymer stabilizers. Both phosphite and thioesters are affected by vulcanization systems, losing most of their activity. PA1 Often an antioxidant from each of the two groups is used in a polymer to form a synergistic combination. By combining the two, they can work hand in hand to destroy both types of destructive radicals. PA1 Actually, such a combination almost always works much better than simply increasing the concentration of a single antioxidant."